Pearl Harbor and the Pacific War
Though aircraft carriers were in service by the early 1930's in the navies of Britain, Japan, and the United States, the Second World War saw the development of the modern conception of a carrier battle group - one or more aircraft carriers escorted by (at least) cruisers and destroyers for anti-aircraft and anti-submarine protection.
The Japanese attack on Pear Harbor began the role of the carrier battle group as the dominant force in naval combat. To strike the American fleet in Hawaii, it sent six aircraft carriers - the Akagi, Kaga, Shokaku, Zuikaku, Hiryu, and Soryu - with over three hundred aircraft to deal a mortal blow to the US Pacific Fleet. Alongside these six carriers were only two fast battleships and a small cluster of escorts. The striking power of this carrier battle group was incredible, and led to the complete destruction of two American battleships with five more put out of action for varying lengths of time.
The dearth of battleships caused by this disaster led to the United States itself adopting the carrier battle group as its primary strike force. The Lexington, Saratoga, Enterprise, Yorktown, Hornet, and Wasp led the US Navy in the first year of the Pacific War, and they fought the first naval battles conducted solely through the medium of carrier based aircraft. Though only the Enterprise and Saratoga survived the war, lessons learned from their use led to the construction of 24 Essex class fleet carriers that carried the carrier battle group into the Cold War.